1. Field of the Invention
The present invention relates to a liquid droplet jetting apparatus for jetting liquid droplets.
2. Description of the Related Art
An ink jet printer has been hitherto widely known, in which an ink-jet head and tanks (ink cartridges) for storing inks are connected by means of tubes, and the inks are supplied to the ink-jet head via the tubes. A flexible tube, which is formed of, for example, a synthetic resin material, is generally used for each of the tubes, for the following various reasons. That is, it is intended that the tube can be freely laid out to secure the degree of freedom of the layout. In another viewpoint, it is intended to avoid the disturbance of movement of the head as thoroughly as possible, which would be otherwise caused by the tubes when the head is moved.
The state of the ink contained in the tube has been hitherto detected for various reasons. For example, an ink jet printer (ink-jet recording apparatus) disclosed in Japanese Patent Application Laid-open No. 5-57905 includes a sensor (optical detector) which detects the presence or absence of the ink and which is provided at an intermediate position of a tube for connecting an ink jet head and an ink cartridge, wherein the exhaustion of the ink in the ink cartridge is detected by means of the sensor.
Japanese Patent Application Laid-open No. 10-157161 discloses an ink-jet printer which is configured to perform the gradational recording by using a plurality of types of inks having different concentrations. The concentration of the ink is raised on account of the evaporation or vaporization of a solvent. An ink concentration detecting section, which detects the increase in the ink concentration, is provided at an intermediate position of a tube for connecting an ink jet head and an ink cartridge. The amount of a dilution liquid (diluent) to be added to the ink is controlled to adjust the concentration of the ink to be supplied to the ink-jet head on the basis of a detection result obtained by the ink concentration detecting section.
On the other hand, it is feared that the air may be mixed in from a connecting portion between an ink jet head and a tube and/or a connecting portion between the tube and a tank to form any bubble. If such a bubble is fed to the head together with the ink, any harmful influence is exerted on the liquid droplet jetting performed by the head. In order to solve this problem, an ink jet printer disclosed in Japanese Patent Application Laid-open No. 2004-188647 has a transmission type photosensor which detects any bubble contained in a tube and which is provided at a connecting portion of an ink-jet head connected to the tube.
As disclosed in Japanese Patent Application Laid-open Nos. 10-157161 and 2004-188647, any bubble is generated in the tube during the ordinary use, and/or the ink contained in the ink is dried to increase or raise the concentration (viscosity) of the ink. In the following description, the expression that “the viscosity of the ink is increased or raised” also includes the meaning of the increase in the concentration of the ink. For example, the air enters through the connecting portion of the tank when the tank (ink cartridge) is exchanged, and the bubble is mixed into the tube in some cases. On the other hand, the flexible tube, which is formed of, for example, the synthetic resin material, generally has the gas permeability. Therefore, if the flexible tube is left to stand for long period of time, then the air is mixed into the tube to generate the bubble in some cases, and/or the solvent of the ink contained in the tube is evaporated to increase the viscosity of the ink in other cases. If the bubble and the viscosity-increased ink contained in the tube as described above are fed to the head, then the abnormality (for example, the nozzle-absence by which the ink droplets are not jetted, or the jetting-curvature by which the ink droplets are not jetted in the right direction) arises in the liquid droplet jetting performed with the nozzle by the head, and the printing quality is deteriorated.
However, in Japanese Patent Application Laid-open Nos. 5-57905, 10-157161, and 2004-188647, the detecting means, which detects the presence or absence of the ink in the tube, the concentration, and/or the presence or absence of the bubble, has the fixed position with respect to the tube. The detecting means can detect only the state of the ink at a certain portion of the tube. Therefore, if the bubble passes through the predetermined detection position by some chance during the period in which the state of the ink in the tube is not detected (for example, during the period in which the power source is turned OFF), it is feared that the bubble may thereafter arrive at the head without being detected.
Further, in Japanese Patent Application Laid-open Nos. 5-57905, 10-157161, and 2004-188647, it is impossible to recognize at what position the bubble and/or the viscosity-increased ink exists in the tube extending over a long distance from the head to the tank, and it is of course impossible to recognize the size of the bubble and the extent of the viscosity increase. Therefore, it is impossible to optimize the discharge operation depending on, for example, the size of the bubble if it is intended to discharge the bubble from the ink supply system ranging to the head by means of any discharge means until the bubble or the like arrives at the head when the detecting means detects the bubble and/or the viscosity-increased ink contained in the tube. For example, the following problems arise. That is, a large amount of the ink (normal ink not subjected to the viscosity increase) is discharged together with the bubble or the like during the discharge operation for discharging the bubble or the like. In another situation, the discharge operation is performed for an unnecessarily long period of time.